1. Field of the Invention
The present invention relates to a helium gas compressing apparatus used in a helium refrigerating machine and more particularly to a helium gas compressing apparatus which is capable of adjusting the pressure difference between gas supplied to the helium refrigerating machine and gas exiting, or returning, therefrom.
2. Description of the Prior Art
An example of a conventional helium gas compressing apparatus is shown in FIG. 2 and generally has a compressor 1 for compressing helium gas. The compressor 1 has a low-pressure suction side 1a connected to a low-pressure discharge side 3a of a helium refrigerating machine 3 via a low-pressure gas passage 2 while a high-pressure delivery side 1b of the compressor 1 is connected to a high-pressure supply side 3b of the helium refrigerating machine 3 through a high-pressure gas passage 4.
In the high-pressure gas passage 4 is installed an oil separator 5, which is connected to the low-pressure gas passage 2 via an oil return path 7.
Further, between the low-pressure gas passage 2 and the high-pressure gas passage 4 are provided two other paths: a path 9 including a pressure retaining valve 8; and a path 11 including a solenoid valve 10. The pressure retaining valve 8 is intended to determine the pressure difference (or braking pressure) between the high pressure in the high-pressure gas passage 4 and the low pressure in the low-pressure gas passage 2. That is, the pressure difference is determined by a preset force of a spring installed in the pressure retaining valve 8. A portion of the gas in the high-pressure gas passage 4 can flow into the low-pressure gas passage 2 through the path 9 incorporating the pressure retaining valve 8 to keep this pressure difference constant.
The path 11 in which the solenoid valve 10 is installed works as follows. Immediately after the helium gas compressing apparatus stops, the solenoid valve 10 is switched from a closed to an open state to increase the pressure at the low-pressure discharge side 3a of the helium refrigerating machine 3 so that oil in the oil separator 5 and in the compressor 1 will not flow back to the helium refrigerating machine 3 through the low-pressure gas passage 2. Oil backflow is also prevented by a check valve 12.
A pressure switch 13 monitors the pressures in the low-pressure suction side 1a and the high-pressure delivery side 1b of the compressor 1. A thermostat 14 monitors the temperature of the gas in high-pressure gas passage 4. Pressure gauges 15 and 16 monitor the pressures in the high-pressure gas passage 4 and the low-pressure gas passage 2, respectively. A safety valve 17 is designed to release excess gas from the high-pressure gas passage 4 in times of emergency.
In the above-described helium gas compressing apparatus, helium gas is introduced into the low-pressure gas passage 2 through a charge valve 18.
Other elements shown in FIG. 2 will be identified in the following description of the operation of the illustrated conventional helium gas compressing apparatus.
With this helium gas compressing apparatus, the helium gas in the form of an oil mist compressed by the compressor 1 to a high pressure is supplied from the high-pressure delivery side lb of the compressor 1 into the high-pressure gas passage 4 and is then cooled by a cooling fan 19 down to a normal temperature. The cooled gas now passes through the compressor 1 to cool the oil therein and is then cooled again by the fan to the normal temperature on its way to the oil separator 5, which is installed in the high-pressure gas passage 4.
In the oil separator 5, the high-pressure helium gas in the form of an oil mist is separated into high-pressure helium gas and oil. The high-pressure helium gas thus extracted by the oil separator 5 is fed through the high-pressure gas passage 4 to an oil adsorber 20 where residual oil contained in the gas is further removed before being supplied to the high-pressure supply side 3b of the helium refrigerating machine 3. The high-pressure helium gas supplied to the refrigerating machine 3 will hereafter be referred to as supply gas and the helium gas returned to the low-pressure gas passage 2 will hereafter be referred to as return gas.
The supply gas fed to the helium refrigerating machine 3 is returned from the low-pressure discharge side 3a of the refrigerating machine 3, as return gas, into the low-pressure gas passage 2. The return gas flows through the check valve 12 and a strainer 21 to the low-pressure suction side 1a of the compressor 1 where it is compressed again into a high-pressure helium gas in the form of oil mist.
The oil separated by the oil separator 5 is passed through a capillary tube 6 to meter or restrict the oil flow to a predetermined amount, which is then fed to the low-pressure gas passage 2, from which the oil flows through the strainer 21 to the low-pressure suction side 1a of the compressor 1 and into the compressor 1.
The pressure retaining valve 8 that determines the pressure difference (braking pressure) between the supply gas in the high-pressure gas passage 4 and the return gas in the low-pressure gas passage 2 operates to allow a part of the high-pressure gas in the high-pressure gas passage 4 to flow into the low-pressure gas passage 2 so that the pressure difference determined based on the spring force of the spring installed in the pressure retaining valve 8 is maintained.
With the conventional helium gas compressing apparatus described above, however, the pressure retaining valve 8, which determines the pressure difference (braking pressure) between the supply gas in the high-pressure gas passage 4 and the return gas in the low-pressure gas passage 2, is operated under control of the spring force of the preset spring installed therein, so that there is no possibility of adjusting the pressure difference. Hence the pressure difference is fixed and it is structurally impossible to change the preset pressure difference from outside.
Thus, the following problem arises with the conventional helium gas compressing apparatus of the above construction. In cannot meet such user demands as making in-service adjustments on the pressure difference in the helium gas compressing apparatus according to power consumption of the apparatus and to specifications involving the refrigerating capability of the helium refrigerating machine. Both the power consumption and the refrigerating capability depend on the magnitude of this pressure difference. Because of the inability to make fine adjustments on this pressure difference, a single apparatus cannot meet varying specifications and power consumptions. In other words, two or more helium gas compressing apparatuses are required to accommodate such demands.
With a gas-driven helium refrigerating machine, in particular, vibrations and impacts occur depending on the magnitude of the pressure difference, or braking pressure. It has therefore been an urgent task to develop a helium gas compressing apparatus capable of making fine adjustment on the pressure difference to alleviate the vibrations and impacts produced during operation.